A molybdenum boride (MoB) coating was prepared on molybdenum (Mo) by an in-situ chemical vapor deposition method. To develop a fuller understanding of the effect of depositional conditions on the growth of MoB coating formed on Mo substrate, this paper studied the phase composition and microstructure of the coating at varied conditions. At the same time, the relationships between the growth of the coating in thickness and depositional conditions were also investigated. The phase composition of the coating, which was prepared using 0.2 wt % boron (B), was composed of MoB and Mo2B phases. The single MoB phase was obtained using a high B content (above 0.2 wt %). The microstructure of the coating showed that MoB coating could be prepared at temperatures over 900 °C. With the increase of temperature and time, the thickness of the coating increased. The growth of the coating was also influenced by the content of NaF. With increasing content, the thickness of the coating first increased, and then decreased. In addition, the prepared coating had a high hardness (3130.85 HV), and the mass change of the coated sample was only 4.92 mg/cm2 after isothermal oxidation at 600 °C for 100 h. The MoB coating could provide a good oxidation resistance for Mo at 600 °C.